Thin film distributed rc network



May 13, 1969 FIG.

w. WOROBEY THIN FILM DISTRIBUTED RC NETWORK Filed 001;. 31, 1966INVENTOR n4 WOROBE V ZMW A T TORNE Y United States Patent US. Cl. 29-5924 Claims This invention relates to thin film structures. Moreparticularly, the present invention relates to a novel thin filmdistributed RC structure.

In recent years, considerable interest has been generated in thin filmRC networks because of the inherent advantages of such structures overnetworks of individual resistors and capacitors, the most importantbeing the relative ease of precisely adjusting the RC product to obtaina desired frequency response by trim anodization of the film resistors.

Unfortunately, the trim anodization technique is not readily applicableto thin film distributed RC networks (which offer several advantagesover the lumped thin film RC network, namely, ease in synthesizing afunction by the elimination of components, etc.) in which a filmcapacitor is superpositioned upon a film resistor due to the fact thatthe superposed counterelectrode prevents trim anodization of theunderlying resistor path.

In accordance with the present invention, this prior art limitation iseffectively obviated by a novel structure comprising a thin filmcapacitor having a low density tantalum counterelectrode of a desiredresistive configuration. The described structure readily lends itself toprecise adjustment of the RC product by trim anodization of the lowdensity tantalum layer, so resulting in the desired frequency response.The novel structures described herein are obtained by preparing a thinfilm capacitor by conventional techniques, the counterelectrodecomprising low density tantalum obtained by cathodic sputteringtechniques. Thereafter, conventional photolithographic methods areemployed to generate the desired resistive configuration.

The invention would be more readily understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing wherein:

FIG. 1 is a cross-sectional view of a substrate with a layer of afilm-forming metal deposited thereon;

FIG. 2 is a perspective view of the body of FIG. 1 after the generationtherein of a desired pattern;

FIG. 3 is a cross-sectional view of the body of FIG. 2 afteranodization;

FIG. 4 is a cross-sectional view of the body of FIG. 3 after depositionthereon of a counterelectrode and the generation therein of a desiredpattern;

FIG. 5 is a cross-sectional view of the body of FIG. 4 after the trimanodization of the top layer thereof; and

FIG. 6 is a plan view of the body of FIG. 5.

With further reference now to FIG. 1, there is shown a substrate uponwhich a metallic pattern is to be produced in accordance with thepresent invention. Preferred substrate materials suitable for thispurpose are glazed ceramics, glasses and so forth.

The first step in fabricating a structure in accordance with theinvention involves cleansing the substrate by conventional techniqueswell known to those skilled in the art. Following the cleansing step, alayer of a film-forming metal 12 is deposited upon a substrate 11 by anyconventional procedure as, for example, cathodic sputtering, vacuumevaporation and so forth, as described by L. Holland in VacuumDeposition of Thin Films, J. Wiley & Sons, 1956. The film-forming metalsof interest herein are those whose oxides are known to be excellentdielectric ice materials and include tantalum, aluminum, niobium,titanium zirconium and hafnium.

For the purposes of the present invention the minimum thickness of thelayer deposited upon the substrate is dependent upon two factors. Thefirst of these is the thickness of the metal which is converted into theoxide form during the subsequent anodizing step. The second factor isthe minimum thickness of unoxidized metal remaining after anodizationcommensurate with the maximum resistance which can be tolerated in thefilm-forming metal electrode. It has been determined that the preferredminimum thickness of the metal electrode is approximately 1000 A. Thereis no maximum limit on this thickness, although little advantage isgained by the increase above 10,000 A.

Following the deposition, a suitable pattern is generated in layer 12 byconventional photoengraving techniques, so resulting in the structureshown in perspective in FIG. 2. Prior to anodizing the structure of FIG.2, a suitable procedure is employed to mask out terminal area 13 such asthe use of a grease.

Thereafter, the structure of FIG. 2 is anodized in an appropriateelectrolyte, so resulting in the formation of a dielectric oxide layer14 on the bottom electrode 12. The voltage at which the anodizing isconducted is primarily determined by the voltage at which the structureis to be operated. Suitable electrolytes for this purpose are oxalicacid, citric acid, and so forth. Backetching and reanodizing may then beemployed for the purpose of eliminating defects in the dielectric film.Following, the grease is removed from terminal area 13, so resulting inthe structure shown in FIG. 3.

The next step in the fabrication of a distributed RC network inaccordance with the invention involves the deposition of a low densitytantalum layer upon the structure of FIG. 3. For the purposes of thepresent invention, the term low density tantalum is defined as tantalumevidencing a density considerably less than the bulk density of 16 gramscm. For the purposes of the present invention, it has been foundnecessary to utilize tantalum evidencing a density less than 14 gramscm. An optimum has been found to correspond with the range of 10- 12grams cm. It has been determined that tantalum of normal density resultsin structures evidencing high leakage currents and numerous shortcircuits. It has been theorized that such behavior is caused by damagedone to the dielectric oxide layer by the impinging high energy tantalumatoms. Accordingly, it is essential that the tantalum film deposited beof low density, that is, produced by means such that essentially nodamage occurs to the anodic oxide film.

It has been further determined that the low density tantalum filmsrequired herein may only be obtained by cathodic sputtering techniquesutilizing sputtering voltages ranging from 8002500 volts and partialpressures of sputtering gases ranging from 10-100 millitorr. Deviationsfrom the noted extrema fail to generate the required glow discharge orresult in the production of normal density tantalum. It will beunderstood by those skilled in the art that in addition to an inert gassuch as argon, reactive gases may be employed in the sputteringreaction, oxygen and nitrogen being prime examples thereof. Immediatelyafter deposition of the low density tantalum counterelectrode, aresistor pattern 15 (FIG. 4) is generated therein by conventionalphotoengraving techniques and terminals 16 defined. Thereafter, thestructure of FIG. 4 is subjected to trim anodization in order to obtainthe desired frequency response, conventional anodization techniquesbeing employed. The resultant structure including oxide film 17 is shownin cross-sectional view in FIG. 5 and in plan view in FIG. 6. In orderthat those skilled in the art may more fully understand the inventiveconcept herein 3 presented, the following example is given by way ofillustration and not limitation.

Example A 1" x 3" glass microscope slide was cleaned with ultrasonicdetergent washes and boiling hydrogen peroxide in accordance withconventional techniques. Thereafter, the substrate was positioned in acathodic sputtering apparatus and a layer of tantalum 4000 A. inthickness deposited at 5000 volts and 300 milliamperes employingconventional techniques. Next, the substrate was subjected toconventional photoengraving techniques to define the desired pattern.Following, the tantalum layer was anodized in a 0.01 percent aqueoussolution of citric acid, a constant current phase of 1 milliampere cm.being employed until a voltage of 130 volts was attained. At this pointthe assembly was left to anodize for 30 minutes at this maximum voltage.

Following the anodization, the assembly was backetched for five secondsat 75 volts in a 0.01 percent solution of aluminum chloride in methanolin order to eliminate defects in the tantalum pentoxide dielectriclayer. Then the assembly was reanodized for 30 minutes at the originalanodizing voltage. Next, low density tantalum was deposited upon theanodized layer by cathodic sputtering techniques at 1500 volts and 300milliamperes with an argon pressure of 35 millitorr. Sputtering wascontinued for 40 minutes, so resulting in a low density counterelectrode1800 A. in thickness. The next step in the fabrication of thedistributed network involved defining the resistor pattern includingterminals in the counterelectrode coating by conventional photoengravingtechniques, thereby resulting in a structure similar to that shown inFIG. 4. The resultant structure evidenced a capacitance of 0.10microfarad Cm. a forward breakdown voltage of 23 volts, a reversebreakdown voltage of 22 volts (breakdown voltage being defined as thevoltage at which the current drawn while charging the device at 1 voltper second reaches twice its initial charge current), and a leakagecurrent of 1.5 X amperes at volts.

The frequency response of the network is next measured typically interms of output voltage/ input voltage against frequency or in terms ofphase difference between output and input against frequency. As thefrequency response will not necessarily be that desired at a givenfrequency,

anodization of the top resistor may be effected until the frequencyresponse measured equals that desired.

While the invention has been described in detail in the foregoingspecification, and the drawing similarly illustrates the same, theaforementioned is by way of illustration only and is not restrictive incharacter. The several modifications which will readily suggestthemselves to persons skilled in the art are all considered to be withinthe scope of the invention, reference being had to the appended claims.

What is claimed is:

1. A method for the fabrication of a thin film distributed RC networkwhich comprises the steps of (a) depositing a layer of a film-formingmetal upon a substrate by condensation techniques, (b) anodizing saidfilm-forming metal whereby there is formed an anodic oxide layer, (c)depositing a layer of low density tantalum evidencing a density lessthan 14 grams cm? upon said anodic oxide layer by cathodic sputteringtechniques, the voltages ranging from 8002500 volts and partialpressures of sputtering gas ranging from 10-100 millitorr, and (d)generating a desired resistor pattern in said low density tantalumlayer.

2. A method in accordance with claim 1 wherein said film-forming metalis tantalum.

3. A method in accordance with claim 1 wherein said low density tantalumis sputtering at 1500 volts with a partial pressure of argon of 35microns.

4. A method in accordance with the procedure of claim 1 wherein thedensity of said low density tantalum is Within the range of 1012 gramscmf References Cited UNITED STATES PATENTS 2,694,185 11/1954 Kodama 3333,109,983 11/1963 Cooper et al. 333--70 X 3,205,555 9/1965 Balde et al.29-620 X 3,239,731 3/1966 Matovich 317-258 3,330,696 7/1967 Ulleey etal.

JOHN F. CAMPBELL, Primary Examiner.

I. L. CLINE, Assistant Examiner.

US. Cl. X.R.

1. A METHOD FOR THE FABRICATION OF A THIN FILM DISTRIBUTED RC NETWORKWHICH COMPRISES THE STEPS OF (A) DEPOSITION A LAYER OF A FILM-FORMINGMETAL UPON A SUBSTRATE BY CONDENSATION TECHNIQUES, (B) ANODIZING SAIDFILM-FORMING METAL WHEREBY THERE IS FORMED AN ANODICOXIDE LAYER, (C)DEPOSITION A LAYER OF LOW DENSITY TANTALUM EVIDENCING A DENSITY LESSTHAN 14 GRAMS CM. 3 UPON ANODIC OXIDE LAYER BY CATHODIC SPUTTERINGTECHNIQUES, THE VOLLAGES RANGING FROM 800-2500 VOLTS AND PARTIALPRESSURES OF SPUTTERING GAS RANGING FROM 10-1000 MILLITRON, AND (D)GENERATING A DESIRED RESISTOR PATTERN IN SAID LOW DENSITY TANTALUMLAYER.